The present invention relates to electric power control systems and, more particularly, to a system for obtaining power supply control voltages for semiconductor switching regulators in which the control voltages can exceed the system input voltage.
Alternating current electric motor propulsion systems typically utilize direct current (DC) electric power to develop alternating current (AC) electric power at a regulated voltage and frequency. DC power may be generated on-board by an internal combustion engine driving an electric alternator subsystem or be obtained externally from an overhead catenary subsystem or from a third-rail system. In each situation, an inverter employing solid-state or semiconductor devices is used to convert the DC to AC power. In some systems, the inverter utilizes power gate-turnoff (GTO) devices. These GTO devices require gate drive circuits to provide current and voltage of relatively precise values in order to assure that the devices are gated into and out of conduction at precise times. In order to achieve this function, the power supplied to the gate drive circuits must be precisely regulated.
One form of power supply for an inverter gate drive circuit uses a two stage converter to generate the precise voltage for the gate drive circuit. A first stage of the converter comprises a boost circuit which boosts a variable input, relatively low DC voltage to a substantially regulated higher voltage. A second stage of the converter comprises an inverter which converts the DC voltage from the boost circuit to an AC voltage, which AC voltage is transformer coupled to the gate drive circuit to achieve galvanic isolation. Both the boost circuit and the inverter utilize pulse width modulation controllers for controlling the on and off times of switching devices in each of the boost circuit and inverter. Further, the switching devices necessitate additional gate drive circuits in the power supply for supplying gate drive signals to the switching devices. Regulated power for these additional gate drive circuits is usually obtained by conventional integrated circuit (IC) devices specifically designed as linear voltage regulators. Such regulators obtained power from the boost circuit and have their ground reference terminals connected to a common ground reference terminal of the boost circuit. One problem with such regulators is that the common ground reference establishes ground loops between the separated power supply stages. Additionally, at high current levels, voltage drops in a ground reference bus coupling the boost circuit to the inverter creates a ground reference voltage difference between the coupled converter stages.